1. Field of the Invention
The present invention is directed to a method for controlling a tomogram acquisition device for acquiring tomograms of an examination subject, of the type wherein reference images of the subject are presented by means of a graphic user interface, and the positions of tomograms to be subsequently acquired are defined by slice position markings entered within the displayed reference images. The invention also is directed to a corresponding control device for a tomogram acquisition device as well as to a tomogram acquisition device having such a control device.
2. Description of the Prior Art
Tomograrn acquisition devices such as, for example, X-ray computed tomography systems and nuclear magnetic resonance tomography systems are primarily employed in the medical field. In most instances, the tomogram exposures serve the purpose of examining body parts or organs of a patient for a later diagnosis. They also are often employed in the context of surgical interventions. For example, German PS 198 46 687 discloses a method wherein a relatively exact three-dimensional presentation of the operation region is first recorded pre-operatively by means of a magnetic resonance apparatus. Moreover, ultrasound image data of the operation region are acquired with an ultrasound head both pre-operatively and at various points in time during the operation. Changes of the region being operated on are determined by comparing the ultrasound image data acquired at the various points in time, and the three-dimensional magnetic resonance dataset is updated on the basis of these changes and displayed. If a magnetic resonance imaging method that that produces an image of sufficient detail cannot be intra-operatively employed, this method makes it possible to generate xe2x80x9cartificial magnetic resonance imagesxe2x80x9d with the assistance of a relatively simple ultrasound acquisition method.
Further, a large variety of tomogram acquisition methods can be employed for the non-destructive examination of arbitrary, other subjects.
When using such tomogram acquisition methods, it is fundamentally desirable for optimally few exposures of the person or article under examination to be acquired so that an unambiguous, dependable examination result is achieved. This is especially important in the medical field since the examination time, what is usually uncomfortable for the patient, and possibly the radiation stress as well, can be reduced in this way. To this end, it is necessary that the positions of the tomograms to be acquired be selected such that the object of the examination within the article or person, for example a specific organ of a patient, is covered in a suitable way in the tomograms.
Particularly when examining a patient, however, the region of interest cannot always be exactly localized in advance from the outside since, first, the exact position of an organ in the body of the patient is dependent on the individual anatomy of the patient and, second, the region of the organ under examination wherein a pathological change that must be examined in greater detail is situated becomes clear only during the course of the examination.
In order to exactly position tomograms, the initially cited control method is currently generally employed. For example, standard tomograms of the test subject or of the body part to be examined, for example the head or the chest area, are first generated as reference images. The acquisition of the reference images usually ensues within the tomogram acquisition device itself. It is also possible, however, to employ tomograms generated with some other device insofar as there is a possibility of suitably positioning the patient under examination in the tomogram acquisition device on the basis of the exterior anatomy. With the assistance of suitable input means, for with a standard computer mouse, a graphics tablet, a keyboard or the like, the operator of the tomogram acquisition device can then set slice position markings within the reference images, for example in the form of section lines and/or projection presentations. In the medical field, a sagittal image, a coronary image and a transverse image are often generated as the reference images. In order to generate a simple plane within the three-dimensional examination subject, a slice position marking in the form of a section line in two of the images and a further slice position marking in the form of a projection presentation in the third image are usually required. Moreover, it is possible to set the size or the thickness of the slices. Usually, a number of slices to be acquired can be immediately marked within the reference images, for example a group of a number of parallel slices, in order to thus cover the region of the structure to be examined in the best way. This method of defining the positions of the tomograms to be acquired by means of a marking in reference images of the subject, which is rather comfortable for the user, is called xe2x80x9cgraphic slice positioningxe2x80x9d (GSP). Further parameters required for the control of the tomogram acquisition device also can be defined for the individual tomograms to be acquired. In a magnetic resonance tomography apparatus, for example, these are the relaxation time TA and the echo time TE, etc., or for an X-ray computer tomography system, the dose to be set, etc. When the operator has set all parameters and optimally covered the measurement region with the planned slices, the measurement can be started by means of what is referred to as a xe2x80x9cmeasurement queuexe2x80x9d. The data of the slice position markings within the reference images are then converted into position data within the examination subject, and the tomogram acquisition device, or the scanner is driven such that the desired images are generated at the corresponding slice positions within the examination subject. The generated tomograms are then stored in an image databank. All measured images are directly available for further slice positioning, i.e. they can in turn be employed as reference images at the user interface in order to enter new slice position markings for further measurements.
Various methods of generating tomograms with the assistance of a graphic slice positioning are described, for example, in German OS 100 48 438 and 195 29 636.
German OS 100 48 438 discloses a method that generates a rotated presentation of the reference image dependent on a command input by a user and generates a spatial presentation of the slices on the picture screen corresponding to the rotation of the reference image. As a result, the spatial orientation of the slices that have been selected and are to be measured is visualized for the user with respect to the reference image of the measured body part of the patient. Particularly in instances of doubly inclined slice groups, the user can understand the actual situation in a simple way and judge whether the planned slices that are presented in the reference images in fact cover the region or the body part to be examined, without the user having to be exceptionally capable of imaging spatial relationships.
German OS 195 29 636 likewise discloses producing an overview exposure of the subject perpendicular to the desired slices and then graphically positioning the desired slices on the basis of the overview exposure. A 3D dataset that covers the prescribed slices is to be subsequently produced. The desired slices are then reconstructed from the 3D dataset and ultimately imaged.
A problem of all of the aforementioned methods occurs when the orientation of the subject changes perpendicularly to the desired image plane over time. In this case, the structures of interest are no longer completely imaged. The image plane therefore must be readjusted in conformity with the motion. A typical example of this is the examination of a heart valve of a patient. The position of the heart valve changes constantly due to respiration and the heart activity. A simultaneous derivation of motion information from the subject of interest for image readjustment is generally not possible in two-dimensional imaging because the structure to be examined, for example a heart valve, is often too small and relatively poorly delimited. Moreover, the motion measurement and the image measurement must be implemented temporally separated, with the motion measurement to be prospectively implemented influencing the image signal. In practice, indirect methods therefore are currently employed that determine the motion component of interest for the structure to be actually examined from the motion of a different structure, for example of a different organ of the patient. A method that is often utilized is referred to as xe2x80x9cnavigator echo techniquexe2x80x9d, wherein the signal from the tomogram acquisition device, and thus the position of a reference structure is acquired, for example, a linear coupling is determined between the current orientation of the reference structure and the structure to be examined. For example, the position of the diaphragm is identified with this method in order to correct the respiratory position of the heart. A disadvantage of this method is that not only the structure to be examined but also the reference structure must be permanently measured in order to determine the dislocation. A part of the measuring time thus no longer is available for measuring the structure that is actually wanted. This problem exists in all methods that make use of a reference structure for the readjustment of the measurement slice.
A method referred to as the xe2x80x9cslice tracking methodxe2x80x9d also is known, wherein the motion of a structure as a function of time is measured in advance and the motion of the measured slice is calculated therefrom in advance. This method has the disadvantage that it can be applied only for certain structures that are large enough for such an examination such as, for example, the diaphragm or the liver of a patient. It cannot be applied in the case of smaller structures such as, for example, heart valves. In such cases, an indirect method must be used wherein the motion of the valve is derived, for example, from the upper myocardium of the heart.
An object of the present invention is to provide an alternative control method for tomogram acquisition devices and a corresponding control device, which allow a simple, time-dependent slice positioning that can be universally applied for various examinations and that, in particular, allow a reliable readjustment of the position of the tomograms, so that small structures under examination can be reliably observed over a longer time span.
This object is achieved in a method according to the invention wherein a sequence of time-dependent slice position markings, i.e. at least two slice position markings, is first set in the reference images, and a time mark is allocated to the individual slice position markings. The time mark can refer to a specific reference point in time or to the time markings of the other slice position markings of the sequence, i.e., for example, relative time intervals are allocated to the individual slice position markings. Using this sequence of time-dependent slice position markings, the positions of the tomograms to be subsequently acquired are determined dependent on the acquisition time of the respective tomogram relative to a reference time, for example relative to a starting point of the measurement or relative to the acquisition time of the previous tomogram.
Due to the possibility of linking the respective slice position markings with a time mark, the operator can interactively specify the position of each image plane, and thus an arbitrary path of a series of exposures within the subject as a function of the time. The operator also can exactly plan a measurement series relative to the time component and thus optimally adapt it to the particular examination. The motion of the structures of interest also can be taken into consideration, so that additional, complicated measurements for the readjustment of the image plane are not necessary.
As used herein xe2x80x9cpositionxe2x80x9d and positioning mean not only the definition of the spatial location of the slice but also its orientation, as well as - in some circumstancesxe2x80x94the shape and the volume of the slice a specific locations, i.e. the position of each volume element. The slices may be planar, plane-parallel slices or may be hyperbolic planes. For simplicity but not as a limitation, the following discussion is based on the positioning of a planar tomogram.
A control device for the implementation of the method employs a standard user interface with a graphic user interface for the presentation of the reference images and with means for setting slice position markings. Moreover, a slice position determination unit is required for determining the position of tomograms of the subject to be subsequently acquired on the basis of the slice position markings in the reference images. For example, the determination of the positions in the subject from the slice position markings can ensue by means of a simple conversion of the graphics data into the positions coordinates in the test subject. Further, an operating unit is required in order to drive the tomogram acquisition device such that tomograms of the subject are recorded at the positions determined by the slice position determination element. For example, the operating unit includes interfaces, D/A converters, etc., for driving the various components of the tomogram acquisition device such as motor actuators or electromagnets, transmission/reception coils for the magnetic resonance signal, etc. Inventively, the control device also includes a setting unit to set a sequence of time-dependent slice position markings in the reference images and to allocate respective time marks to the individual slice position markings of the sequence. The slice position determination unit operates such that, on the basis of the sequence of time-dependent slice position markings, it determines the positions of tomograms to be successively acquired dependent on an acquisition time of the respective tomogram relative to a reference time.
A series of tomograms to be acquired at various points in time can be determined in a single reference image, or in two or three reference images that show the subject from various points of view at a specific point in time. In a preferred exemplary embodiment, however, a sequence of time-dependent reference images is generated first that show the subject at different relative points in time relative to a reference time, or at defined time intervals relative to one another. On the basis of the time-dependent reference images, the operator is able to acquire information about how a specific tomogram should be optimally positioned at a specific relative point in time and can accordingly set the slice position marking.
It is preferred to set a slice position marking in a time-dependent reference image that is automatically allocated to the relative point in time of the reference image itself, as the time mark. Preferably, exactly one slice position marking is set in each of the time-dependent reference images. The control device includes an allocation unit for this purpose that allocates the relative points in time of the respective reference image to the slice position markings as the time mark.
Preferably, the sequence of time-dependent reference images is generated during an event that corresponds to the event during the later measurement of the tomograms for the examination. This can be periodically reoccurring events such as, for example, the respiratory or heart motion of the patient, or singly triggered events such as, for example, a swallowing motion. For example, the patient can first swallow a contrast agent for examining the esophagus during a swallowing motion. A sequence of reference images is acquired during this first swallowing event. The operator of the tomogram acquisition device then sets the slice position markings in these reference images in order to define the relative points in time and the relative region of the esophagus in which a tomogram is to be generated. Subsequently, the actual examination is implemented during a repetition of the swallowing event.
The sequence of time-dependent reference images preferably shows a specific, moving structure of the subject to be acquired such as, for example a heart valve in different positions, i.e. at least one component of the motion direction of the structure of interest should lie in the image plane of the time-dependent reference images. The slice position marking is then set in each of the time-dependent reference images such that a slice of the subject that encompasses this structure of the subject is marked. In an examination of a heart valve, this means the slice position marking is set such that the marked slice includes the heart valve in each of the time-dependent reference images.
In the simplest case, the tomograms are respectively acquired at points in time in the following measurement series that each correspond exactly to a time mark of a slice position marking. The tomogram is then acquired exactly at the position corresponding to the slice position marking.
Preferably, however, a time-dependent positioning function and/or a reference table is generated from the sequence of time-dependent slice position markings. The positions of the tomograms at arbitrary relative acquisition times then can be determined on the basis of this time-dependent positioning function or the reference table (look-up table). The data of the sequence of time-dependent slice position markings can form the supporting points of the function or the entries of the look-up table, or they can form supporting points for generating a more complex look-up table. The positioning function can be implemented with standard mathematical methods, for example with arbitrary, suitable fit and interpolation methods with which functions are generated on the basis of supporting points. This can ensue segment-by-segment with respect to the time. Likewise, the look-up table can be completed using suitable interpolation methods when being generated between the individual supporting points. Given employment of a look-up table, known interpolation methods can be accessed for determining positions for acquisition times that lie between the entries of the look-up table.
The relative points in time of the individual tomograms of the later measurement series can be selected independently of the temporal position of the reference images that are employed. As a result, it is possible to generate only a small number of reference images with large temporal spacings for the inventive graphic slice positioning and to subsequently acquire a series of tomograms that is relatively dense in terms of time in the actual examination. The overall examination duration can be shortened, and the number of reference images can be reduced to a minimum, particularly in an examination with an X-ray computed tomography apparatus, and thus the radiation exposition time of the patient can be shortened.
The control device preferably is able to implement an exposure series, with the starting point of the exposure series serving as the reference time for determining the positions of the individual tomograms dependent on the respective acquisition time.
The occurrence time of a specific event occurring in or at the subject can be selected as the starting time. Such an event can be, for example, the aforementioned initiation of a swallowing event or, in the case of a heart examination, a specific event within the heart motion, for example the occurrence of the typical r-wave or s-wave in the EKG of the patient being examined. A triggering of a simple MR exposure by the r-wave of an EKG is described, for example, in the article xe2x80x9cECG-Triggered Snapshot MR Imaging of the Heartxe2x80x9d, Computers in Cardiology, Proceedings 23-26 September 1990, pp. 381-384, by Y. Liu, S. J. Riederer, D. G. Brown, R. C. Wright, A. E. Holsinger, R. C. Grimm and R. L. Ehman.
To this end, the control device preferably includes an arrangement for determining the event at or in the subject as the reference time, for example a suitable measuring instrument. Alternatively, a signal of an external measuring instrument that is connected to the control device via an interface can be employed. The measuring instrument can automatically generate a trigger signal given occurrence of the event that triggers the start of the exposure series.
The exposure series can be periodically repeated. In an examination at the heart, for example, several image series can be acquired that are each implemented within an r-r interval measured in an EKG, whereby triggering can also ensue at a new r-blip each time. It is not necessary that the tomograms always be implemented at the same relative points in time with respect to the reference time; rather, tomogram exposures can be generated at individual points in time in each image series, with the position of each tomogram being re-determined dependent on the relative acquisition time.
The inventive control device can be largely realized in the form of suitable software on a computer having an adequate computing power. This can be a standard computer with appropriately adapted interfaces for controlling the tomogram acquisition device.
In particular, the slice position determination unit that defines the exact positions of the tomograms within the subject on the basis of the slice position markings in the reference images and calculates the positions dependent on the relative acquisition time can be installed on a processor of the computer in the form of software modules.
Likewise, the components of the user interface that edit the image data for presentation on the graphic user interface and that convert the commands of a mouse, a keyboard or similar input devices into data in order to set the markings on the graphic user interface, can be realized on this processor, or on a separate computer that has the user interface as a terminal.
Further, the devices for the automatic control of the tomogram acquisition device for acquiring a series of reference images or the later, actual series of examination images, the unit for registering the information about the relative point in time of the reference images, the allocation unit which allocates the relative point in time of a reference image to a slice position marking as time mark, as well as the unit for determining a positioning function and/or a reference table from the reference data can be realized in the form of software modules.
A modification of an existing control device of conventional tomogram acquisition device is therefore possible in a relatively simple way to program it to operate according to the inventive method.